The present invention relates to a method for the preparation of a copolymer of vinyl chloride and an N-substituted maleimide compound. More particularly, the invention relates to a method for the preparation of a copolymer of vinyl chloride and an N-substituted maleimide compound having, as compared with conventional polyvinyl chloride resins, greatly improved heat resistance and thermal stability and usefulness as a material of various kinds of polyvinyl chloride-based molded articles such as tubes, corrugated boards, flooring boards, window sashes and the like.
As is well known, polyvinyl chloride-based resins are widely used in the form of molded articles not only in the household applications but also in a variety of industrial applications by virtue of their high resistance against aging to ensure long durability and excellent mechanical properties along with the low costs in the mass production of the resins.
One of the disadvantageous properties of these polyvinyl chloride-based resins, which are mostly prepared by the method of suspension polymeriation, is their relatively low heat resistance and thermal stability readily to cause thermal deformation at an elevated temperature so that the molded articles prepared from the resin can hardly be used in an application in which the article may have a chance to be heated at an elevated temperature. Accordingly, many attempts and proposals have been made to obtain a polyvinyl chloride-based resin having improved heat resistance and thermal stability. One of the ways hitherto undertaken with this purpose is to copolymerize vinyl chloride with one or more of other comonomers along a guide line that certain copolymers of vinyl chloride may have higher heat resistance or thermal stability than a homopolymeric polyvinyl chloride resin. N-Substituted maleimide compounds are among the comonomers used for such a purpose (see, for example, Japanese Patent Publications No. 41-9551 and No. 44-12433). Copolymers of vinyl chloride and an N-substituted maleimide compound, however, have a defect that the particle size distribution of the resin particles prepared by the suspension polymerization is usually less uniform than the homopolymeric resins and the porosity of the resin particles is small so that problems are caused in the molding process of the resin that proceeding of the so-called gelation with a plasticizer is slow and the molded articles may be sometimes non-uniform with appearance of colored spots on the surface.